mount: tell why FUSE support is unavailable

docs/installation.rst

@@ -118,6 +118,12 @@ are available on the releases_ page for the following platforms:
 
 ARM binaries are built by Johann Bauer, see: https://borg.bauerj.eu/
 
+.. note:: ``borg mount`` only works if the binary was built with FUSE support
+          (third-party binaries might lack it) and your OS has FUSE installed
+          (e.g. the ``fuse3`` package on Linux, macFUSE on macOS).
+          If ``borg mount`` fails, the error message will tell for each FUSE
+          implementation why it could not be loaded.
+
 To install such a binary, just drop it into a directory in your ``PATH``,
 make borg readable and executable for its users and then you can run ``borg``::
 

src/borg/archiver/mount_cmds.py

@@ -18,10 +18,12 @@ def do_mount(self, args):
         """Mounts an archive or an entire repository as a FUSE filesystem."""
         # Perform these checks before opening the repository and asking for a passphrase.
 
-        from ..fuse_impl import llfuse, has_mfusepy, BORG_FUSE_IMPL
+        from ..fuse_impl import llfuse, has_mfusepy, BORG_FUSE_IMPL, fuse_import_errors
 
         if llfuse is None and not has_mfusepy:
-            raise RTError("borg mount not available: no FUSE support, BORG_FUSE_IMPL=%s." % BORG_FUSE_IMPL)
+            msg = "borg mount not available: no FUSE support, BORG_FUSE_IMPL=%s." % BORG_FUSE_IMPL
+            msg += "".join(f"\nimport of {impl} failed: {err}" for impl, err in fuse_import_errors.items())
+            raise RTError(msg)
 
         if not os.path.isdir(args.mountpoint):
             raise RTError(f"{args.mountpoint}: Mountpoint must be an **existing directory**")

src/borg/fuse_impl.py

@@ -12,13 +12,16 @@
 hlfuse: types.ModuleType | None = None
 llfuse: types.ModuleType | None = None
 
+# import failures by implementation name, see issue #8657
+fuse_import_errors: dict[str, str] = {}
+
 for FUSE_IMPL in BORG_FUSE_IMPL.split(","):
     FUSE_IMPL = FUSE_IMPL.strip()
     if FUSE_IMPL == "pyfuse3":
         try:
             import pyfuse3
-        except ImportError:
-            pass
+        except Exception as err:
+            fuse_import_errors[FUSE_IMPL] = str(err)
         else:
             llfuse = pyfuse3
             has_llfuse = False
@@ -30,8 +33,8 @@
     elif FUSE_IMPL == "llfuse":
         try:
             import llfuse as llfuse_module
-        except ImportError:
-            pass
+        except Exception as err:
+            fuse_import_errors[FUSE_IMPL] = str(err)
         else:
             llfuse = llfuse_module
             has_llfuse = True
@@ -43,8 +46,8 @@
     elif FUSE_IMPL == "mfusepy":
         try:
             import mfusepy
-        except ImportError:
-            pass
+        except Exception as err:  # mfusepy raises OSError (not ImportError) if libfuse is missing
+            fuse_import_errors[FUSE_IMPL] = str(err)
         else:
             hlfuse = mfusepy
             has_llfuse = False

src/borg/helpers/misc.py

@@ -36,19 +36,22 @@ def sysinfo():
         msgpack_version = ".".join(str(v) for v in msgpack.version)
     except:  # noqa
         msgpack_version = "unknown"
-    from ..fuse_impl import llfuse, BORG_FUSE_IMPL
-
-    llfuse_name = llfuse.__name__ if llfuse else "None"
-    llfuse_version = (" %s" % llfuse.__version__) if llfuse else ""
-    llfuse_info = f"{llfuse_name}{llfuse_version} [{BORG_FUSE_IMPL}]"
+    from ..fuse_impl import hlfuse, llfuse, BORG_FUSE_IMPL, fuse_import_errors
+
+    fuse_mod = hlfuse or llfuse
+    fuse_name = fuse_mod.__name__ if fuse_mod else "None"
+    fuse_version = getattr(fuse_mod, "__version__", "") if fuse_mod else ""
+    fuse_version = " %s" % fuse_version if fuse_version else ""
+    fuse_errors = "".join(f" {impl}: {err}." for impl, err in fuse_import_errors.items()) if not fuse_mod else ""
+    fuse_info = f"{fuse_name}{fuse_version} [{BORG_FUSE_IMPL}]{fuse_errors}"
     info = []
     if uname is not None:
         info.append("Platform: {}".format(" ".join(uname)))
     if linux_distribution is not None:
         info.append("Linux: %s %s %s" % linux_distribution)
     info.append(
         "Borg: {}  Python: {} {} msgpack: {} fuse: {}".format(
-            borg_version, python_implementation, python_version, msgpack_version, llfuse_info
+            borg_version, python_implementation, python_version, msgpack_version, fuse_info
         )
     )
     info.append("PID: %d  CWD: %s" % (os.getpid(), os.getcwd()))

src/borg/testsuite/fuse_test.py

@@ -5,7 +5,11 @@
 errno, which is what we check here without mounting anything.
 """
 
+import builtins
 import errno
+import importlib
+import os
+from unittest.mock import patch
 
 import pytest
 
@@ -126,3 +130,33 @@ def test_mfusepy_getxattr_broken_acl():
     with pytest.raises(hlfuse.FuseOSError) as excinfo:
         getxattr(ops, "/file", "system.posix_acl_access")
     assert excinfo.value.errno == errno.EIO
+
+
+def test_fuse_import_errors_recorded():
+    """A FUSE impl failing to import must not crash borg and must record why, see #8657."""
+    import borg.fuse_impl
+
+    real_import = builtins.__import__
+
+    def failing_import(name, *args, **kwargs):
+        if name == "mfusepy":
+            raise OSError("Unable to find libfuse")  # mfusepy raises OSError, not ImportError
+        if name in ("pyfuse3", "llfuse"):
+            raise ImportError(f"No module named '{name}'")
+        return real_import(name, *args, **kwargs)
+
+    try:
+        with (
+            patch.object(builtins, "__import__", failing_import),
+            patch.dict(os.environ, {"BORG_FUSE_IMPL": "mfusepy,pyfuse3,llfuse"}),
+        ):
+            fuse_impl = importlib.reload(borg.fuse_impl)
+            assert not fuse_impl.has_any_fuse
+            assert fuse_impl.hlfuse is None and fuse_impl.llfuse is None
+            assert fuse_impl.fuse_import_errors == {
+                "mfusepy": "Unable to find libfuse",
+                "pyfuse3": "No module named 'pyfuse3'",
+                "llfuse": "No module named 'llfuse'",
+            }
+    finally:
+        importlib.reload(borg.fuse_impl)  # restore the real state